The present invention relates to a multi-layer film based on polyamides, which comprises at least two layers and has improved physical properties as compared to prior art films, and which may be used for packaging purposes.
From U.S. Pat. No. 3,762,986 multi-layer films are known which are based on various polyamides and are made up of structures according to the systems A/B/A, A/B, or B/A/B. In this case, the A layers represent polyamides having melting points below 200.degree. C., e.g., polyamide 11 or polyamide 12, whereas the B layers represent polyamides having melting points above 200.degree. C., e.g., polyamide 6, polyamide 6,6 or polyamide 6,10.
These composite films exhibit a low permeability to moisture, but their permeability to, e.g., oxygen is relatively high. The permeation values (in cm.sup.3 /m.sup.2 .multidot.d.multidot.bar for 40 .mu.m thick films) range, e.g., for polyamide 12 from about 300 to 380 and for polyamide 11 from about 160 to 200. For many applications this permeability to oxygen is still too high to prevent adverse effects on the goods packed due to oxidation. Problems may also be encountered concerning the adhesion values between the individual layers, as the films are produced by coextrusion.
U.S. Pat. No. 3,093,255 discloses the preparation of mixtures of polyamides (e.g., of polyamide 6 or polyamide 6,6) and polyethylene, which may, for example, be used for manufacturing films or for blow molding bottles. Although these products have a low permeability to gases, measurements have shown that the permeability to oxygen still exceeds 500 cm.sup.3 /m.sup.2`.multidot.d.multidot.bar for 40 .mu.m thick films. As indicated above, these high permeation values cannot be tolerated in many applications. This United States patent does not mention the manufacture of composite films. It is only stated that the bottles produced may be coated with polyvinylidene chloride, thus reducing their permeability. However, such coatings must be regarded as critical for ecological reasons, since toxic vapors are generated when the shaped articles are destroyed in combustion plants.
In German Offenlegungsschrift No. 25 51 023 a polymer mixture is described which is composed of 60 to 90 parts by weight of polyamide, 40 to 10 parts by weight of polyethylene, and 1.8 to 8.0 parts by weight of a plastic graft copolymer. In this case, it is merely mentioned that mixtures of this kind may be used for the manufacture of wire insulations or shaped articles.
The journal "Kunststoffe", Volume 65, Number 3, pages 139 to 143 reports investigations conducted on plastic mixtures of polyamide 6 and high-pressure polyethylene; but it is not possible to infer from this report any suggestion for producing films or even composite films using these mixtures.